Audio frequency signal transmission circuit



Oct.. 13, 1942.

P. H. THOMSEN AUDIO FREQUENCY SIGNAL TRANSMISSION CIRCUIT Filed Aug. 3o, 1940 2 Sheets-Sheet 1 SF2 QQ Se QQQN 8m. SN QQ Qw QN Q \\M} \M. o ../I\\ f m NN n m n Nt n n n n 4 m IlvlvIulIvlllll ...Jil ||IJ Oct 13, 1942- P. H. THoMsI-:IN l 2,298,987

AunIo FREQUENCY SIGNAL TRANSMISSION CIRCUIT FII-eid Aug. so, 1940 2 sheets-sheet 2 a, s, I a/ua asm/af pf su? fequency ['r fl a/ 759 Insel?.

X f eguency Patented Oct. 13, 1942 UNITED STATES PATENT OFFICE AUDIO FREQUENCYzzAL TRANSBISSION Paul B. Thomsen; Silver Spring, Md.

Application August 30, 1940, Serial No. 354,903

` 13 claims. (cina-171i The invention relates to a frequency controll sfxstem for audio frequency ampliiiers and the l' e. t f

An object of this invention is to provide highly rlexible control over the audio frequency response or tone characteristics of an ampliner.

It is also an object of the present invention to provide a frequency response control system for an audio frequency signal transmission circuit which provides continuously adjustable proportioning between the high, intermediate, and low components of'signals transmitted therethrough.

.It is also a further object of the invention to provide a high, intermediate, and low frequency control system for an audio frequency signal circuit, said'system comprising combinations ofresistor and capacitor elements having high, intermediate, and low frequency peak response curves, the characteristic curves of which overlap, without the introduction of phase shift resulting in serious modification `of lthe desired response curve. i

It is also an additional object of the present controls a certain amount through the gear and rack coupling, thus introducing into the system the desired frequency response characteristic. This exible bar, insofar as the operator is concerned, is a curve or a graph, which,- when changed in shape, affects the frequency response curve of the entire amplifier system.

Referring to the drawings briefly,

Fig. 1 illustrates a schematic diagram of two signal paths or channels of a multiple channel audio-frequency ampliiier;

Fig. 2 is a graph showing the relative frequency response and phase relations of each signal path or channel;

Fig. 3 is a View showing frequency response curves obtained in a fifteen channel system; and

Fig. 4 is a View of a flexible bar or key for manipulating the variable control of each channel to obtain a desired overall frequency charinvention to provide a frequency response confrequency signal from the circuit through a coml bination of resistor-capacitor networks .having peaked response when operated individually. thus making it possible to attenuate thelow, inter'. mediate, and high frequencies with respect to the control or channel' supplying the amplification.

Other objects and advantages of the invention will appear from the following description and the claims. 1

The system of my invention allows a condition of adjustment for maximum attenuation or rise of audio-frequency signals in not only the upper and low frequency portion of the audio range, but the middle, as well, whereby the desired frequency response characteristic may be introduced. This may or may not be accomplished through the use of individual or ganged controls. For example, my system makes it possible for the controls to be coupled to a ilexible bar, the length of which represents the complete `frequency range coveredby the system and the width or contour of which is shaped in accordance with the frequency response curve desired. The individual controls are in this case coupled to the :exible bar by .means of gear and rack or other suitable arrangements, and the flexible bar may be bent or cut into shape to actuate each of the acteristic.

Referring to Fig. 1 in detail, reference numeral I0 designates an audio-frequency amplifier tube having a grid, an indirectly heated cathode, and an anode.- A potentiometer circuit indicated as Il is connected by a variable contact to the grid of tube I0 and by one of its terminals to the bias potential circuitv I2 and ground. This latter potentiometer terminal is also connected to the sheath of the coaxial cable I3, the inner conductor of which is connected tothe other terminal of said potentiometer. The bias potential circuit l2 is connected between the cathode of tube I0 and ground to which is also connected the negative terminal of the anode current supply so that suitable bias potential is developed by the potential drop caused by the anode-cathode current ilowing through the resistor of circuit' l2.

The tube l0 may be in the form of a duel triode with one of the triode sections thereof connected to feed a number of the networks and the other section thereof connected to feed other networks described in succeeding paragraphs of this specification. Or this tube may be in the form of any other conventional amplier tube. A plate loadcircuit I4. and coupling networks l5 and I6, are connected between the anode oi' tube lil and the input of tubes I1 and It. Each of the coupling networks l! and II consists of a condenser and a resistor connected in series with a parallel connected cpndenser and resistor, the latter resistor being in the form of a.i potentiometer with its adjustable contact connected to a succeeding amplifier tube. Thus the circuit l5 consists of a condenser I9 connected in series with a resistor 20 and a parallel connected conn `I "and resistance 22. lThe variable contact of the potentiometer resistance 22 is connected to the grid of the tube I1 and the lower terminal of this resistance is connected to the around and to the circuit 2l which in turn is connected to the cathode of tube I1. The circuit I i consists of condenser 2l connected in series with resistance 2l -and the parallel connected condenser 26 and potentiometer resistance 21. The variable output of potentiometer resistance 21 is connected to thegrid of tube Il. This circuit is similar to the circuit Ii except that its response is peaked at a dinerent frequency as described in following paragraphs of this specification.

Additionalnetworks like Il and Ii are connected to the conductors 2S, 2| and 3| so that additional frequency selective paths may be employed. It will be seen from an examination of the curves cf Fig. 3 that fifteen channels were employed in the arrangement providing those curves. The potentiometers 22 and 21 are employed for level control of the channels associated therewith and a ,variable control such as one of these pctentiometers is provided to each of the peak response networks. The tubes I1 and II are electronic mixers connected to the a`i'ore-l said networks. Resistors 22 and 22 in connection with resistors 24 and 2l, respectively provide suitable coupling to the loudspeaker and the output circuit through the coupling condenser ll. With this arrangement, each of the signal channel tubes operates into its respective plate or anode circuit load with the least number of parts. All

of the tubes are provided with their respective self-bias resistor circuits 23 and 2l, having suitable audio frequency by-pass capacitors. These bias circuits are connected between the tube cathodes and the chassis or ground which may be considered as the negative or B minus anodepotential supply connection. 'Ihe positive anode current supply terminal is connected to the resisters 22 and Il.

The resistors 3l and Il are employed to prevent interaction between the channels so that one or more of the tubes employed as electronic mixers will not place a load on the other tubes. If all of the tubes were connected in parallel to a common resistor, the use of one of the tubes or channels would require this tube to modulate the plate currents of al1 of the other tubes which would result in amplitude distortion of the signal. By using the separate resistors u and Il for each tube, the .tubes individually work or feed into a load circuit of around 25,000 ohms or any other value selected.

A condenser 2B is provided between the junction of resistors I4 and I! and the output circuit. This condenser 3l may be connected direct to a succeedingamplifier tube, for example, the grid of such a tube, or it may be connected to one or more loud-speakers by means of a coaxial cable. the sheath of which is connected to ground to complete the circuit to the cathodes of tubes I1, I8. Where desired a matching circuit consisting of a resistance 3l shunted by a condenser may be connected in series with the coaxial conductor 31 and the condenser II and a resistance I0 may be connected across the terminals of the coaxial conductor.

A simple resistance capacitor netwo is'we/ ployed in each channel. The frequency ponse characteristic oi' each network is rather broad; although it is peaked enough to produce the desired results ss is shcwfm rig. 2. curve A of this gure shows hcw,theratio ofthe input to ascesa? output voltage changes with change in frequency, and curve B shows the change in phase relations of networks, such as Il and Ii, Fig. 1. It will be noted that the phase angle changes over a straight line B to C from a leading angle of about 60 degrees to a lagging angle of about B0 degrees. The phase angle varies as the frequency varies from the peak and there are no harmonic or secondary peaks outside of the peak frequency value. As can be leen in Pig. 2, the peak response is rather broad, although it does have a general shape which is similar to the resonance curve of the regular inductance-capacitance tank circuit employed in tuned circuits; unlike such circuits it is not accompanied by harmonic peaks which is a very important feature of my circuit arrangement.

A careful study of the characteristics of the network Il will disclose the reason for the peak in the response curve'as the frequency of the input voltage is varied. The upper section consistlng of condenser I s and resistance u can be thought' of as a high-pass series section whose transmission falls ci! gradually as the frequency is decreased, and the lower section consisting of condenser 2| and resistance 22 can be thought of as a low-pass shunt element which offers an increase in impedance with decrease in frequency and hence allows a larger voltage to be developed across it.

As the frequency of the voltage across the input side oi the circuit is decreased from a high value, the impedance cf the lower section tends to increase untilthc value of the series impedance increases more rapidly than the shunt impedance of. `the bottom section. At the point where the two impedances increase at the same rate the peak response or the peak voltage occurs, since the total voltage across the two sections is constant, and since the two impedances are increasing at the same proportional rate.

As this point is passed, the reactance of condenser 2I becomes somewhat small with respect to resistance 22 so that the resistance makes up the greater portion cf the circuit. But, in the upper element the resistance and capacity are in series so that the reactance of the condenser I! continues to add to the ristance 2l. Therefore, the voltage across the lower branch falls of! as the frequency is increased past the peak point or the point at which the reactances of the upper and lower sections increase at the same rate.

Referring to Fig. 2 again, the point at which the output voltage at E2 will be the greatest percentage of EI and that which it will have the same phase as EI, will occur at the frequency determined by the formula:

Where R and E' are the resistances 2l and h 22 (Fig. 1) and Cand C' condensers I! and 2I, respectively.

Careful examination of the equation will disclose that the peak frequency is inversely proportional to capacitance, instead oi' being inversely proportional to the square root cfce1 pacitance. as in the caseof the circuit. employing an inductance coil instead of a resistor.

It can, therefore, be shown that the frequency response characteristic of this resistive-capacitive combination is equal to that shown by curve A in Fig.f2 and by the individual curves of Fig. 3. The components I9 and 20 ofl'er an increasing impedance as the frequency vofthe signals decreases, thereby passing the high frequency signals. The impedance of the circuitr including the components 2I and 22 increases as the signal frequency decreases, causing the lower frequency signal potential to build up across this'circuit ever, the voltage across condenser 2| and likewise the voltage across 22, the potentiometer in Fig. l, is reduced more'and more by the drop of voltage across condenser -I9 and as a result, the extreme low frequency signals do not appear across resistance 22 or on the grid of tube I1. For the increasing and highest frequencies, it will be found that condenser I'! andresistance 2l offer a lowering impedance where condenser Il will have very little voltage drop across it Since condenser 2I also offers low reactance to the higher frequencies, substantially all of the voltage drop will take place across resistance 20 again indicating that there is a drop in the voltage applied to the grid of tube II. It may be stated that the maximum voltage will appear on the grid of tube I1 when the reactance of condenser Il equals resistance 20 and when the reactance of condenser 2l equals resistance 22,

when the capacities are equal` and the resist-vk ances are also equal. Byselec'ting equal values for the capacities I 9 and 2|, as well as resistances 20 and 22 it is possible to have the above condition oc'cur at any desired frequency, thus giving a peaked response with sumcient attenuation of the high and lower frequencies to introduce the required frequency response characteristics for my system. For example, with resistor elements 20 and 22 of 50,000 ohms each, and with condenser elements of .015 microfarad, the response curve shown in curve L3 of Fig. 3 was obtained. Fig. 3 shows the actual response curves of a working model f or 15 individual peak response resistor-capacitor networks, such as. networks I5 and Il, Fig. i, the peak response frequencies in this model being 23, 37, 60, 96, 154, 245, 394, 630, 1000, 2560, 4100, 6550, 10,300, and 16,480 C. P. S., `as shown by the curves Ui, L1, U2, Le, Ua, Io, U4, Li, Us, I s, Us, Ls, U1, In, and Us, respectively, in Fig. 3. each peak frequency is substantially 1.6 times its next lower or preceding frequency. It will be noted that when all of the signal paths or channels are in use the response characteristic is substantially flat. When the channels for curves U1 and Us are used for equal maximum peak gain, the least amount of gain will occur at and adjacent the frequency of 650 C. P. S. This point may be moved up or below this frequency by either raising or lowering the gain of either channel U1 or Ua. That is, the level and thus be applied to the grid of tube I'I. Howjustment ofthe controls attached to the respective channels for these frequencies.

Examination of Fig. 3 will disclose that the average frequency response variation from one channel to the other is about 5 db. rIfhis variation is sufficient to change the tone of an audio frequency signal a noticeable amount. `In other words, it is possible to vary the frequency re-` to specify to his listeners that he is going to raise or lowerthe response within a given frevquency range. That is, the level of either 'the low, intermediate or high frequencies or any frequencies between these ranges may `be altered without effecting a change in the level range of the other channels.

This invention has many applications where i changes in tone quality are questionable 'or' analyses of such apparent changes are desirable. The

invention also finds application inp'ublic address systems, broadcast stations, sound movie film production and reproduction, as in theaters, radio' receiving sets and other audio frequency circuits, where a wide range of frequencies is being passed and it may be desirable iso-quickly modify, the tone to the satisfaction of the audience. It is remarkably easy fora group of technically-minded m n and musicians to pass their opinions on the elity when usingmy system of tone control as they can specify the general frequency range in which the level may be raised or lowered in Inthiscase of either of the two channels may be modified. Y

an attempt to duplicate the original tonal quality.

The apparatus of my invention may be used in determining the desired frequency response curve necessary to give good fidelity by connectingthe-apparatus shown in Fig. 1 using 15 chan nels into the audio frequency circuit, and adjusting the various potentiometer controls for best ndelity. The frequency response of the audio frequency amplifier is then measured and plotted and an equalizer giving the same curve is introduced into the amplifier being analyzed. The apparatus with the 15 controllable channels is then removed to determine the fidelity curves of other amplifiers or circuits.

In Fig. 4 is illustrated a view of the flexible bar II which is made to correspond to the frequency response curve desired. This flexible bar or key may be placed on the front of the panel of the cabinet housing the multiple channel unitdescribed in this specification. The potentiome ters of the frequency selective networks are mounted behind this Apanel and correspond in what may be termed "frequency positions illustratedin Pig.' 4. l'ior example the potentiometers l'and I'I'shown in Pig. 1 may correspond to the first two potentiometers shown in Fig. 4. Each of these potentiometersand each of the succeeding potentiometers 42 to 54 inclusive is provided with a small 'gear pinion, such 'as 5i attached to theslmft thereof and engaging the gear rack il which isformed integral with a small rod l1 adapted to engage the top surface of the flexible bar 4I. Thus. when the gear rack rods of lll of orme the potentiometers are in engagement with the top surface of the flexible bar curve 4|, the signal levels of the frequency selective channels are all adjusted so that the output of the multiple channel arrangement corresponds to the curve 4|. In this way any desired output may be obtained from -the multiple channel arrangement simply by forming the curve 4i in accordance with the output desired.

Various modifications of this invention may be made within the spirit and scope thereof and therefore I do not desire to limit the invention to the exact details descrbed and shown except insofar as they may be defined by the claims.

What I claim is as follows:

1. In an audio frequency amplifier in which the amplification in different portions of the audio frequency spectrum may be selectively controlled, the combination comprising: an input circuit, a plurality of electrical networks each adjusted to respond to a slightly different audio frequency band throughout the audio frequency spectrum, each of said electrical networks consisting of a resistance and a condenser connected in series and a resistance and condenser connected in parallel, said parallel connected resistance and condenser having a terminal connected to said series connected resistance and condenser, connections for connecting said plurality of electrical networks to said input circuit, an output circuit connected to said electrical networks to receive signal energy from each of said networks corresponding to the frequency bund to which the corresponding network is selective to substantially prevent coupling between the different ones of said networks, and variable means for adjusting one of said resistances in each of said networks for adjusting the signal strength impressed from the correspondingvone of said electrical networks on said output circuit substantially without affecting the frequency characteristic of any of said networks.

2. In an audio frequency amplier in which the amplification in different portions of the audio frequency sp^ctrum may be selectively controlled, the combination comprising: an input circuit, a plurality of resistance and reactance electrical networks each adjusted to respond to a slightly different audio frequency band so that said networks taken altogether cover the audio frequency range of the amplifier, connections for connecting said plurality of electrical networks to said input circuit, an output circuit connected to said electrical'networks to receive signal energy from each of said networks corresponding to the frequency band to which the corresponding network is selective, variable means for adjusting the signal strength impressed from different ones of said electrical networks on said output circuit, and a preformed member for engaging all of said variable means for adjusting all of said variable means to produce a frequency response of the amplifier that may be altered in accordance with the shape of said preformed member.

3. In an audio frequency amplifier in which 4 the amplification in different portions ofthe connected to said electrical networks to receive 75 signal energy from each of said networks corresponding to the frequency band to which the corresponding network is selective, means for varying the voltage derived from different ones of said electrical networks and impressed on said output circuit, and a preformed member for engaging all of said last mentioned means for adjusting the voltages derived from all of said networks in accordance with the shape of said preformed member.

4. In an audio frequency amplifier in which the amplication in different portions of the audio frequency spectrum may be selectively controlled, the combination comprising: an input circuit, a plurality of electrical networks each adjusted to respond to a slightly different audio frequency band throughout the audio frequency spectrum, each of said electrical networks consisting of a resistance and a condenser connected in series and a resistance and condenser connected n parallel, said parallel connected resistance and condenser having a terminal connected to said series connected resistance and condenser, connections for connecting said plurality of electrical networks to said input circuit, an output circuit connected to said electrical networks to receive signal energy from from each of said networks corresponding to the frequency band to which the corresponding network is selective, means for adjusting one of said Y resistances in each of said networks for adjusting the signal strength impressed from different ones of said electrical networks on said output circuit, and a preformed member for engaging all of said last ment'oned means for adjusting the voltages derived from all of said networks in accordance with the shape of said preformed member.

5. In an audio frequency amplifier adapted for the amplification of a multiplicity of frequencies throughout the audio frequency spectrum selectively, the combination comprising: an input circuit including an amplifier tube having a cathode and a grid connected to said input circuit and an anode, a plurality ofrfrequency selective signal paths each consisting of a resistive-capacitive network adjusted to pass a different range of frequencies, connections for connecting said plurality of signal paths in parallel toA said anode, said signal paths being adjusted so that the frequency characteristics of said amplifier may be controlled in different portions of the audio frequency spectrum by varying the output of different ones of said signal paths, an output circuit, and gang control means for controlling the signal strength fed from said plurality of signal paths to said output circuit.

6. In an audio frequency amplifier adapted for the amplification of a multiplicity of frequencies throughout the audio frequency spectrum selectively, the lcombination comprising: a plurality of frequency selective signal paths each consisting of a resistive-capacitive network adjusted to pass a different range of frequencies, an input circuit, an output circuit, connections for connecting said plurality of signal paths between said input circuit and said output circuit, a potentiometer included in each of said signal paths for varying the output of different ones of said signal paths, and adjustable key means for engaging each of said potentiometers to produce a signal frequency response output curve deter- 'mined by the contour of said key,

7. In an audio frequency amplifier, the combination with an interstage coupling circuit, of

a resistive-capacitive network comprising: a plurality of parallel signal paths, each path having frequency response characteristics peaked on a frequency slightly different from the other paths so that each path aids in covering the entire frequency range of the amplifier system, a potentiometer voltage control device for each of said paths for providing adjustment of the frequency response level of each of said paths, a member having a contour corresponding to the frequency response curve desired for the amplifier, and means for connecting selected ones of said potentiometers with selected sections of said member to adjust said potentiometers to produce the desired frequency response in said amplifier.

8. Audio frequency amplifier apparatus in which the amplification in different portions of the audio frequency spectrum may be selectively controlled, comprising: an input circuit, a plurality of electrical networks each individually peaked to slightly different audio frequencies, the'frequency of one said network being approximately 1.6 times the frequency of the preceding network throughout the audio frequency spectrum, each of said electrical networks consisting of a series circuit including a resistance and a condenser connected in series and a parallel circuit including a resistance and a condenser connected in parallel, said series circuit and said parallel circuit in each of said electrical networks being connected in series across said input circuit, an amplifier for each of said networks, connections for connecting said last mentioned resistance in each of said networks to the input of .1

the corresponding amplifier to substantially prevent coupling between the different ones of said v networks, and an output circuitffor connectingthe amplifiers of all of said networks, whereby the output of different ones of said networks may be varied without affecting the frequency characteristic thereof or of the others of said networks so that the desired frequency-gain characteristic for the amplifier is obtained simply by varying the output of said different networks.

9. In an audio frequency amplifier adapted for selectively amplifying a multiplicity of frequency bands throughout the audio frequency spectrum, the combination comprising: an input circuit including an amplifier tubel having a cathode and a grid connected to said input circuit and an anode, a plurality of frequency selective signal paths each consisting of a resistivecapacitive network peaked at a predetermined different audio frequency, the peak frequency characteristics of said signal paths being disposed at predetermined intervals of not more than 100 cycles at the lower audio frequencies and not more than 2000.cycles at the higher vaudio frequencies throughout the audio frequency spectrum to produce a substantially uniform response throughout said spectrum when all of said signal paths are employed at substantially the same level, an output circuit, connections for connecting said plurality of signal paths in parallel between said output circuit and the circuit of said anode, and means for varying the output level of each of said signal paths without varying the frequency characteristics of the individual ones of said signal paths, said last mentioned connections including means for substantially isolating the outputs of said signal paths from each other to prevent varying one signal path from affecting the other signal paths so that a desired frequency-gain characteristic may be obtained.

10. In an audiofrequency amplifier, the combination of an amplifier tube circuit, a resistivecapacitive network comprising: means providing a number of parallel signal paths in a shunt signal circuit connected to said first named circuit, each path having peaked frequency response characteristics arranged at small intervals of not more than 100 cycles at the lower audio vfrequen cies and not more than 2000 cycles at the higher audio frequencies adjacent to each other over the audio frequency spectrum so that each of said paths covers a portion of the frequency range of the entire amplifier system, said resistive-capacitive networks having substantially equal signal output levels, and a potentiometer voltage control device for each of said paths for adjusting the output level of each of said paths to control the overall frequency response of said amplifier, and means for connecting said networks so that varying the potentiometers thereof has substantially no frequency selection effect in any of said networks so that the frequency-gain characteristic of the amplifier may be predetermined.

l1. An audio frequency amplifier apparatus in which the amplification in different portions of the audio frequency spectrum may be selectively controlled, comprising: an input circuit, a plurality of electrical networks each predominantly responsive to a frequency approximately 1.6 times the frequency to which the preceding of said networks is responsive whereby the whole audio frequency spectrum is covered thereby, each of said networks comprising a pair of condensers of approximately equal size connected one on each side of a resistor across said input circuit and another resistor of a size approximately equal to said first resistor connected across one of said condensers, said last resistor having a variable contact, an electronic mixer tube for each of said networks, said variable contact of each of said network resistors being connected to the input of said corresponding mixer tube, a coupling impedance for each of said electronic mixers for connecting the outputs of said mixers together to a common output circuit, the outputs of said electrical networks fed to said ,electronic mixers being controlled by said variable contacts and each ci said variable contacts controlling the output in the audio frequency respouse corresponding to only one audio frequency peak to which the particular electrical network is adjusted.

12. An amplifier according to claim 1, said 'series connected resistance in each of said networks being substantially equal to the parallel connected resistance in the same network, and the capacitance of the series connected condenser in each of said networks being substantially equal to the capacitance of the parallel connected condenser in the same network.

13. An amplifier according to claim 4, said series connected resistance in each of said :net-k works being substantially equal to the parallel connected resistance in the same network, and the capacitance of the series connected condenser in each of said networks being substantially equal to the capacitance of the parallel connected condenser in the same network.

PAUL H. rHoMsEN. 

